Interior Trim Work Basics

We show you how to make crisp, sharp corners and tight joints when installing door trim, window trim and a three-piece baseboard. With a few basic carpentry tools and a little patience, you can trim out a room in a weekend. With a little practice you can master the two key trim techniques, mitering and coping.

Gather your tools and materials

Get set up and ready to start

Clear the room you want to trim out and make your cuts in the middle of the floor. Put a fan in the window if dust is a problem.

In this article we'll show you the basic steps for installing a wide trim around a door and window, complete with mitered corners. We'll also show you how to put in a built-up baseboard made from a combination of three types of moldings.

You'll need several key tools to do a first class job. Use a power miter SAW for clean angle cuts. A power miter saw (Photo 2) vastly simplifies the job because it allows you to make incredibly accurate cuts in a matter of seconds. Even professionals admit they couldn't do the same quality of work without one. If you don't have a power miter saw, you can rent one.

TIP: If you're renting a miter saw, cut several scrap pieces to get the feel of the tool before cutting your trim.

Buy a good-quality blade for your miter saw. The more teeth a blade has, the crisper the cut. Choose a blade with a minimum of 40 teeth, although I prefer a blade with 80 teeth. It leaves a cut that's as smooth as glass, making it well worth the investment.

You'll also need a coping saw. This saw (Photo 18) has a narrow blade and tiny teeth that allow you to cut tight curves; it's available at any hardware store.

TIP: Avoid that annoying trip to the hardware store in the middle of your project—pick up a couple of spare blades in case you break one.

The only other tools you'll need are basic carpentry tools: a sharp pencil, a tape measure and a combination square. A wood file also is a must for fine-tuning joints (Photo 20). Use a round file, called a rattail file, for fitting tight curved profiles, and a combination flat/half round for all other trim (Photo 19). In this project, we predrilled our nail holes. If you have more than one room to trim, consider using an air-powered finish nailer to speed up the process.

If sawdust isn't a problem, cut your trim in the room where you plan to install it (Photo). This will reduce legwork and save a lot of time. Set up the miter saw in the middle of the floor with plenty of room on either side. Cover the floor with a tarp to prevent scuff marks and scratches. Use blocks the same height as the miter saw table to support long lengths of trim.

Prefinishing saves a ton of time

Once you get your trim home, sand and stain or paint the trim before you install it. To be perfectly honest, this part of the project isn't much fun. But it's a lot easier to finish trim before installation, working on sawhorses, than afterward, lying on the floor with sandpaper and a staining rag. Plus you can do a better job of sanding and finishing the pieces. After you've installed the pieces, be prepared to touch them up a bit. Do all the finish work in a well-ventilated area.

We applied a Salem maple stain to our trim and three coats of low-sheen varnish to protect it. Dispose of any staining rags carefully to avoid spontaneous combustion: Open up the oil-soaked rags and hang them up until they're completely dry. Then dispose of them in the trash.

Figure A: Trim Terms

Technical terms can be confusing. Here's a guide to trim carpenter lingo

Figure A: Trim terms

Caution

A miter saw is powerful and loud. Be sure to keep your hands well away from the blade and wear hearing protection and safety glasses when using it.

Video: How to Trim a Door

Travis Larson, senior editor at The Family Handyman, will show you how to trim a door in about 10 minutes per side. The first time you do it, it will take a longer but once you get the hang of it, you'll be trimming out doors like a pro. If you are already a pro, you can still pick up some tips.

Project 1: Case a doorStep 1: Fit one side casing

Photo 1: Mark the reveal

Mark a reveal line 3/16 in. from the edge of the jamb with a combination square. Use a sharp pencil and position the marks in the corners and about every foot along the jamb edge.

Photo 2: Cut test pieces

Cut two 12-in. long test pieces of casing at opposite 45-degree angles on the power miter box to check the fit of your casing on the door jamb.

Photo 3: Check the miter

Hold the test pieces on the reveal marks to check the fit of the miter in the corner. If the joint is even slightly open at the top or bottom of the miter, adjust the angle on the miter saw slightly, recut both pieces and check the fit again. Take your time—you may be surprised how tight you can get the joint to fit.

Photo 4: Cut the side casing

Cut the side casing about 1/2 in. overlong and hold it in place on the door jamb along your reveal marks. Use a sharp pencil to transfer the top reveal mark from the head jamb to the side casing. Then cut the miter at the angle you established with your test pieces.

Photo 5: Predrill and tack

Hold the side casing in place. Predrill nail holes every 12 to 16 in., using a drill bit about 1/32 in. smaller than the nail size. Stay 1 in. away from the ends to avoid splitting. Drive 4d finishing nails into the jamb and 6d finishing nails into the wall.

Start with the casing around the doors and windows. The first few times you install casing, we suggest drawing light lines (called “reveal lines”) 3/16 in. from the edge of the jamb to align the casing to the door jamb and windows (Photo 1). With experience, you'll skip this step and simply “eyeball” the reveal when you put up the trim.

Check the miters at the corners with two 12-in. sections of casing cut at exactly 45 degrees (Photo 3). Even though the corners should be a perfect 90 degrees, often they're not. In addition, if the jamb sticks out or is set back slightly from the wall, a 45-degree miter cut won't fit tight. By holding the test pieces at the corner you can see exactly how your casings will fit. If you see a gap, adjust the saw slightly and cut both pieces at the new angle (Photo 3).

TIP: Make small adjustments. Even one-quarter of a degree makes a big difference.

If the angle of the miter is accurate but a gap still appears along the face, the pieces are probably tipping back against the wall. Cut or file the backside of the miter (back cut). This allows the joint to fit tight on the face of the miter. Don't worry about taking too much off the back; it won't be visible. Mark and cut the first side casing at the angle you established with your test pieces (Photo 4). Always cut the pieces a little long and check the fit; the power miter saw gives you the ability to cut very slight amounts off with a high degree of accuracy. When the inside angle of the miter lines up with the reveal mark on the top of the jamb, tack the casing in place.

Don't split that perfect miter—predrill nail holes.
With maple, oak and other dense wood, predrilling your nail holes in the casing is a must (Photo 5). Even with a softwood like pine, I prefer to predrill to avoid splitting a perfectly fit piece. Use a 1/16-in. bit for 4d nails, 3/32-in. for 6d nails and 1/8-in. for 8d nails.

TIP: Use a nail with the head snipped off as a substitute for a drill bit. The same size nail you're driving works best.

Tack the casing into the jamb first, then to the wall. Wait until you've fit all the casings before you drive the nails in completely in case something doesn't fit right and you have to remove the trim to recut it. We used 4d nails for the jamb and 6d nails for walls. If you're using thicker casing, increase your nail size one increment, using 6d nails in the jamb and 8d nails in he wall. Your nails should penetrate the studs and the jamb at least 3/4 in.

TIP On wood with a strong grain pattern, place your nails in the dark portion of the grain to make them less noticeable.

Project 1: Case a doorStep 2: Install the top and second side

Photo 6: Miter and mark the top casing

Cut the corresponding angle on the top casing, leaving the opposite side at least 1 in. overlong. Check your miter at the second corner with your test pieces, and adjust if necessary to fit tight. Then mark the opposite corner on the top casing, cut it and tack it up.

Photo 7: Mark the second side casing

Cut the second side casing about 1 in. overlong. Then hold the casing backward and parallel to the door jamb. Make a mark where the edge of the side casing intersects the upper edge of the top casing. Cut the side casing about 1/32 in. overlong. Slide the casing into place. Check your fit, and then trim it to its final length. Once the miter fits, nail the casing in place.

Photo 8: Predrill the corners

Align the miters and predrill a 1/16-in. hole for 3d finish nails, one from the top and one from the side. Hold a piece of cardboard against the wall to prevent marring the wall while drilling.

Photo 9: Nail the corners

Drive the nails into the casing gradually, alternating between the two nails so the miter doesn't slide out of alignment. Use cardboard again between the wall and the hammer to avoid marring the wall.

Photo 10: Set the nails

Tap the nails just below the surface with a hammer and a nail set.

With your first corner fit perfectly, set the top casing aside and check the second corner with your test pieces the same way you did the first. Once you have established the angle of the miter, hold your top casing in place and transfer the reveal mark from the side jamb to your top casing (Photo 6). Cut the miter, check your fit and tack in place.

TIP: Match your pieces of wood so the grain pattern and color are similar at the joints.

Then mark and cut the second side casing, leaving an extra 1/32 in. for fitting purposes (Photo 7). Slide the casing into place parallel to your reveal marks and check your fit. If the miter is tight and the length is a little long, trim a hair off the bottom at a 90-degree angle until you get a perfect fit.

When your miters fit perfectly, “pin” the corners (Photos 8 and 9) to help align the two casings and keep the joint tight. Use your finger to press the casings flush with each other. You may have to slip a small shim behind one of the casings to align them. Next predrill the corners for 3d finishing nails, one from the top and one from the side (Photo 8). If your casing is less than 1/2 in. thick, you'll have to predrill the corners before tacking the casing up. Finally, work around the door, driving the nailheads slightly below the surface with a hammer and a nail set (Photo 10). Nail sets are sold in various sizes; choose one that matches the size of the nailhead you're using. Set the nails deep enough to hold nail putty: A good rule of thumb is half the diameter of the nailhead.

Project 2: Case a window

Photo 11: Begin with the top casing

Trim a window using the same techniques as we showed for a door. However, cut and tack the top casing first. Then fit the sides and finish with the bottom.

Photo 12: Miter the bottom last

Fit one miter on the bottom, then overlap the opposite miter and mark it. Cut the miter 1/8 in. overlong, slide the casing into place and trim it down gradually while checking the fit.

There are two basic ways to trim a window. One way is to “picture frame” the window, so that all four corners are mitered to 90 degrees. This method is common on most newer homes, especially with casement windows. The second way is to install a stool and apron. Basically, this is a small ledge (a stool) at the bottom of the window with a piece of casing (an apron) under it. This method is normally found in older homes and is more often used on double-hung windows.

Trim a window using the same techniques as with a door. Mark your reveal lines, use test pieces to check your corners, and transfer the reveal lines to the casings for cutting and nailing. When you “picture frame” a window, however, install the top casing first (Photo 11), then the two sides, and finally, the bottom. Fitting the bottom is the toughest part, because you have to fit both corners at once (Photo 12). But if you use your test pieces and always cut the casing a little long, you shouldn't have any problem. Cutting the piece long allows you to adjust the miters if you have to. Once the miters are tight, gradually trim a small amount off with your miter saw until you have the proper length.

Trim out a window with a stool in the same order as you would a door, but with a few added steps. Install the stool first, then one side, the top and the other side. Install the apron under the stool last.

Project 3: Install baseboardStep 1: Nail up the large base

Photo 13: Cut all the base extra long

Rough-cut all the baseboards about 2 in. overlong and lay them in place around the perimeter of the floor. Start with the longest wall, cut the first piece to length and nail it into the studs with 6d finishing nails. Remember to predrill your holes. Continue around the room, cutting the inside corners off at 90 degrees and butting them together.

Close-up of Photo 13

If you have to splice two pieces on a wall, use a “scarf joint.” Cut a 30-degree angle on each piece; if the joint opens slightly, this angle will hide the crack. Select pieces with similar grain color and pattern so the joint is less visible. Always locate a splice over a wall stud.

Photo 14: Test fit miters

Miter two 1-ft. test pieces about a foot long and press them tightly against the outside corners to determine the correct angle. Adjust the miter saw to get a tight fit.

Photo 15: Cut miter to length

Hold the baseboard in place and mark the backside at the corner. Now cut the piece to length at the predetermined angle for an exact fit.

Photo 16: Cut and nail second piece

Cut the second piece to length at the same angle. Predrill, glue and nail the outside corner.

Begin by using a stud finder to locate the studs, and mark their location on the wall with a narrow piece of painter's tape (Photo 13). You can pull the tape off without leaving a mark. Rough-cut the baseboard about 2 in. overlong and lay the pieces along the wall. Install the longest section first and work away from the ends until you reach an opening or door. This ensures that the last cut will be a simple 90-degree cut.

In general, measure and cut each piece about 1/16 in. overlong to ensure a tight fit. If you don't have a piece of baseboard long enough to cover the entire wall, splice two pieces with a “scarf joint” (see Close-up of photo13). Bow the casing slightly to fit between the walls and press it into place. This ensures a nice, tight fit. But don't force the piece in. Trim a bit off and try the fit again. Continue around the room butting the inside corners at 90 degrees. When you come to an outside corner, use test pieces to find exact angles (Photo 14).

TIP: If you're adding a base cap, as we are, overcut the miter slightly, leaving the backside slightly open (Photo 16). The front side will be tight and the cap will cover the gap.

Nail the pieces in place using 6d finishing nails. You can also put a small amount of glue at the miters and cross-nail them with 4d finishing nails. But remember to predrill to avoid splitting the ends (Photo 16).

Corner Fitting Trick

If the drywall tapers in at the bottom of the wall or stops short of the floor, simply drive a screw at the bottom of the wall and turn it in until the head of the screw is at the same plane as the main wall. The head will prevent the baseboard from tipping in.

Project 3: Install baseboardStep 2: Nail the cap and shoe

Photo 17: Cope inside corners of base cap

Run the base cap in the same order as the baseboard. To cope the inside corner of the base cap for a tight fit, first cut one end at a 45-degree angle as if you were cutting an inside miter.

Photo 18: Cut the profile with coping saw

Use a coping saw to cut along the profile left by the miter. Angle your coping saw back slightly (back cut) to get a tighter fit on the face of the profile.

Photo 19: Test fit the cope

Check the fit against the adjoining base cap in the corner.

Photo 20: Fine tune with a file

Trim with a file as necessary. After fitting the cope, measure the cap for length, cut the other end and nail it up.

Photo 21: Add the base shoe

Install the base shoe the same as the base cap, coping inside corners and mitering outside corners. Where the shoe meets the door casing, cut at a 30-degree angle the portion that sticks out. Predrill and nail the base shoe to the baseboard with 4d finishing nails.

Photo 22: Finish with filler

Set the nails and fill the holes with colored putty to match the wood stain. We mixed two shades of putty together to get a good color match. Press the putty into the holes with your finger and wipe the excess off with a cloth.

Install the base cap pieces in the same order as the baseboard. However, because base cap has a curved profile, you can't butt the inside corners. Instead, make a “coped joint” by cutting off one piece square and cutting the adjoining piece to match the profile of the molding (Photo 19). Just follow the steps in Photos 17 – 20 and you'll find it's easier than it looks. (Actually, coping is kind of fun once you get the hang of it.) Practice a few times on scrap pieces to get used to it.

Install the base shoe last. Base shoe is usually used on hard-surface floors to conceal any irregularities or gaps between the floor and the baseboard. Even if your baseboards fit perfectly tight to the floor, you can install the shoe to add another dimension to your trim. Install the shoe the same as the cap, mitering outside corners and coping inside corners (Photo 21). Be sure to nail the shoe into the baseboard, not the floor, so it won't pull away from the baseboard when the flooring expands and contracts. Finally, set all your nails and fill them with putty. We couldn't find a putty color to exactly match the stain we chose, so we mixed two shades together (Photo 22).

How to Shop for Trim

Stock trim is available in a wide range of styles from most lumberyards and home centers. We chose a relatively wide (3-1/4 in.) beaded casing to go around our doors and windows. It's 11/16 in. thick. We used three components to create the base: a 1/2-in. x 3-1/4 in. “hook strip,” a 9/16-in. x 1-3/8 in. base cap, and a 7/16-in. x 3/4-in. base shoe (see “Trim Terms,” above). Combine other standard trim types to create wider and more detailed shapes.

If you don't find a trim style you like or you're trying to match a molding in an older house, look in the Yellow Pages under “Millwork.” Millwork shops can custom-produce almost any type of trim from most species of wood. Custom work, however, comes at a price; be prepared to pay as much as three times the cost of stock moldings, plus setup charges. In addition, you may have to wait four to eight weeks.

Most trim is made of solid wood or medium density fiberboard with a wood veneer. Oak, pine, birch, maple and poplar are the most common types available. We chose maple for our project.

Sometimes you can find trim made of various types of plastic, most often prefinished in white, brown or simulated wood. This trim is far more stable than wood but cuts much the same, if not easier. What it lacks, however, is the warmth and varied grain pattern you can only find in real wood.

Purchase your trim in lengths long enough to cover each wall. If you can't purchase the trim in long enough lengths, don't worry. In the Close-up of Photo 13 above we show you how to splice two pieces to cover the length of a long wall.

Stock trim is available in a wide range of styles from most lumberyards and home centers. We chose a relatively wide (3-1/4 in.) beaded casing to go around our doors and windows. It's 11/16 in. thick. We used three components to create the base: a 1/2-in. x 3-1/4 in. “hook strip,” a 9/16-in. x 1-3/8 in. base cap, and a 7/16-in. x 3/4-in. base shoe (see “Trim Terms,” above). Combine other standard trim types to create wider and more detailed shapes.

If you don't find a trim style you like or you're trying to match a molding in an older house, look in the Yellow Pages under “Millwork.” Millwork shops can custom-produce almost any type of trim from most species of wood. Custom work, however, comes at a price; be prepared to pay as much as three times the cost of stock moldings, plus setup charges. In addition, you may have to wait four to eight weeks.

Most trim is made of solid wood or medium density fiberboard with a wood veneer. Oak, pine, birch, maple and poplar are the most common types available. We chose maple for our project.

Sometimes you can find trim made of various types of plastic, most often prefinished in white, brown or simulated wood. This trim is far more stable than wood but cuts much the same, if not easier. What it lacks, however, is the warmth and varied grain pattern you can only find in real wood.

Purchase your trim in lengths long enough to cover each wall. If you can't purchase the trim in long enough lengths, don't worry. In the Close-up of Photo 13 above we show you how to splice two pieces to cover the length of a long wall.